(i) Technical Field
This invention relates to an optical apparatus.
(ii) Related Art
A hologram recording and reproducing apparatus records and reproduces digital data by means of recording and reproducing, as a hologram, signal light in which binary digital data “0 and 1” etc., is digitally imaged as, for example, “bright and dark”. In the case of a Fourier transform hologram, signal light is subjected to Fourier transform by means of a lens and the Fourier-transformed light is irradiated to an optical recording medium together with reference light interfering with the recording light to generate an interference fringe, and the generated interference fringe is recorded in the optical recording medium as a hologram. To reproduce data, reference light is irradiated to the optical recording medium, diffracted light is received at a photo detector, and a digital image is reproduced to obtain digital data.
To enhance the recording capacity of an optical recording medium, a method of multiply recording a hologram in the thickness direction of the optical recording medium has been proposed. In this case, it is necessary to shift the focal position of recording light and reference light at high speed and precisely in the thickness direction of the optical recording medium.